This invention concerns a method of surface treatment for forming in a stable manner on an aluminum or aluminum alloy surface a film of which the principal component is aluminum fluoride.
Aluminum and its alloys are often used in applications such as heat exchangers, for example, where the metal comes into contact with moisture. In many heat exchangers the construction involves a very narrow fin spacing since such devices are designed in such a way as to maximize the heat releasing or cooling surface area as much as possible in order to improve the heating or cooling effects of the heat exchanger. As a consequence of this, moisture in the atmosphere condenses out on the surface of the heat exchanger and more precisely in the gaps between the fins when the heat exchanger is being used for cooling purposes. The condensed water readily forms spherical droplets as the surface of the fins is hydrophobic in nature and this tends to block up the gaps between the fins, the ventilation resistance is increased and the heat exchanging efficiency is reduced. Furthermore the water droplets which have collected in the gaps between the fins are dispersed by the air blower of the heat exchanger and the water is readily displaced to the water droplet receiver which is established in the bottom of the heat exchanger and this is disadvantageous in that the region in the vicinity of the heat exchanger becomes contaminated with water. On the other hand when the heat exchanger is used for central heating purposes frost may form on outdoor mechanisms during the winter season with a consequent reduction in the thermal efficiency. As a result the heat exchanger is run in reverse from time to time to heat up and defrost the outdoor mechanism. It is essential that the defrosting operation should require only a short period of time for its effective completion from the point of view of the function of the air conditioning system. The provision of hydrophilic fins is effective for the rapid removal of the water droplets which form as the machine is being defrosted. For these reasons the surface of a heat exchanger is surface treated to render it hydrophilic in nature and to improve its wetability with water so that blockages do not occur between the fins as a result of the water droplets which are retained in the gaps between the fins in the heating part or the cooling part of a heat exchanger. However if a treatment is simply given to improve the wetability of the surface the corrosion resistance for example becomes inadequate and in most cases an anticorrosion treatment is essential, especially in the case of heat exchangers which are made of aluminum.
Known methods of providing hydrophilic surfaces on heat exchangers include (1) methods in which a macromolecular resin skin film which contains silica particles, calcium carbonate or a surface active agent is formed on the surface, (2) methods in which water glass, lithium silicate or colloidal silica etc. is coated on top of an anodic oxidation skin film, a baymite skin film, a resin skin film or a chromate formed skin film, and (3) methods in which water glass, lithium silicate or colloidal silica etc. is coated directly onto the surface of the metal.
However mixed skin films consisting of a resin skin film and particles of silica or calcium carbonate which are solid hydrophilic particles as mentioned under (1) above do not form sufficiently hydrophilic surfaces readily since the surfaces of the solid hydrophilic particles are covered with the resin, while with resin films which contain surface active agents the surface active agent tends to be washed out from the resin by water and it is difficult to achieve lasting hydrophilicity in this way. The methods which involve coating with water glass, lithium silicate or fine silica particles mentioned in (2) and (3) above do provide a hydrophilic surface but the material is only poorly attached to the surface and is easily removed, particularly in areas where too much has been attached to the surface, and this is also disadvantageous in that the stripped solid forms a powder which is subsequently dispersed. Furthermore some of the water glass or lithium silicate etc. is dissolved in the water which condenses on the heat exchanger and this collects at the bottom of the fins and dries out when the air conditioning unit is turned off and this is disadvantageous in that a powder which is dispersed when the air conditioning unit is restarted is formed. The substances which are effective for providing a hydrophilic surface such as fine silica particles, calcium carbonate, water glass, lithium silicate etc. can be attached to the surface only with difficulty; the surface treatment for providing a hydrophilic surface is difficult; there are problems with the formation of excess skin film in places where the treatment liquid collects; the skin filming treatment has to be carried out in very small batches; and there is a further disadvantage in that as a result of these factors it is not possible to provide adequate hydrophilicity.
This invention is intended to overcome the disadvantages described above and the aims of the invention are to provide a method of surface treatment for aluminum and aluminum alloys which forms a uniform film which is firmly attached to the surface of the metal, in which the treatment solution has a long life and which moreover provides a good hydrophilic surface on the metal surface which is effective in practical terms.